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76
CHAPTER 17
17.8
Assuming that, on average, each aluminum atom contributes one electron, the density of
charge carriers is the same as the number of atoms per cubic meter. This is
A
A
density
N
n
mass per atom
M N
M
ρ
==
=
,
or
( )
( )
( )
23
3
6
3
3
28
3
6.02
10
mol
2.7 g cm
10 cm
1 m
6.0
10
m
26.98 g mol
n
×
×
The drift speed of the electrons in the wire is then
()
4
28
3
19
6
2
5.0 C s
1.3
10
m s
6.0
10
m
1.60
10
C
4.0
10
m
d
I
v
neA
−
−−
=
×
××
×
17.9
(a) The carrier density is determined by the physical characteristics of the wire, not the
current in the wire. Hence,
n
is
unaffected .
(b) The drift velocity of the electrons is

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